It is known that particulated metal oxides may be produced by oxidizing volatile metal halides in a vaporized state. In such vapor phase oxidation processes, a volatilized metal halide is caused to be reacted with oxygen, air or like oxygen-containing gas to yield the corresponding metal oxide and gaseous halogen. Exemplary of such processes are those involving the vapor phase oxidation of titanium tetrachloride to produce pigmentary titanium dioxide and by-product chlorine. Generally, the economics of these vapor phase oxidation processes are such that, when operated on a commercial scale, it is desirable to recover the by-product halogen gas produced and contained in an off-gas effluent stream for use in other stages of the process such as, for example, the halogenation stage of the process.
In the vapor phase oxidation of titanium tetrachloride to produce pigmentary titanium dioxide, one method that is employed to recover the by-product chlorine gas is to first quench the hot, gaseous effluent stream from the oxidation stage, after separation of the pigmentary titanium dioxide therefrom, with a cool stream of recycle chlorine gas. The quenched effluent stream then is filtered, to remove any remaining particulate titanium dioxide suspended therein, and scrubbed with a sulfuric acid solution to remove the unreacted titanium tetrachloride. The resulting scrubbed effluent stream, containing chlorine and oxygen gases, then is compressed and cooled for reuse. Generally, the recovered by-product chlorine gas is employed, in part, to quench further gaseous effluent from the vapor phase oxidation reaction and, in part, to provide a source of chlorine for the preparation of titanium tetrachloride feed for use in the process. One drawback to this method is that the unreacted titanium tetrachloride removed by scrubbing of the gaseous effluent with sulfuric acid is permanently lost.
Another method for recovering and reusing by-product chlorine gas from a gaseous effluent resulting from the vapor phase oxidation of titanium tetrachloride to produce pigmentary titanium dioxide is disclosed in U.S. Pat. No. 3,526,477. According to this patent, the method involves selectively absorbing chlorine from a part of reaction off-gases with titanium tetrachloride to obtain a solution of chlorine dissolved in said titanium tetrachloride. A portion of this solution is recovered and recycled to a chlorination zone. The remainder of the solution then is stripped to recover the dissolved chlorine by itself which then is recycled to the chlorination zone. One advantage of this process is disclosed to be that a portion of the off-gases from the vapor phase oxidation reaction, which off-gases contain oxygen, can be added directly to a chlorination zone since the mixture of titanium tetrachloride and dissolved chlorine gas has an endothermic effect on the chlorination reaction. Other processes have been proposed for the recovery of the chlorine contained in the gaseous effluents resulting from the vapor phase oxidation of titanium tetrachloride, including, for example, U.S. Pat. Nos. 3,526,477 and 3,650,695.